1. Field of the Invention
The present invention relates to a fiber amplifier that amplifies an input optical signal in an optical communication system, and more specifically, to an amplified spontaneous emission (ASE) reflector-based gain-clamped fiber amplifier that maintain a certain gain characteristic even when the intensity of the input optical signal varies.
2. Discussion of Related Art
An optical amplifier, such as a semiconductor optical amplifier and a fiber amplifier, which is an optical device that amplifies an input optical signal, is necessary for compensating for optical loss during transmission through optical fiber or various optical devices used in the field of optical transmission and networking.
However, communication quality in an optical network is not good due to the drawback that the degree of amplification of the conventional optical amplifier varies according to the intensity of the input optical signal. Thus, a gain-clamped fiber amplifier has been developed to solve this problem.
An all-optical gain-clamped fiber amplifier in which a gain is optically clamped using a laser cavity does not have a complicated signal process for clamping the gain. Laser oscillation occurs when a loss and a gain generated from the cavity are the same, and once oscillating, amplitude of population inversion of a gain medium is clamped. The gain of the fiber amplifier is proportional to the amplitude of the population inversion and a length of the gain medium, so that when the laser oscillation occurs, the gain of the amplifier is clamped. When an optical signal is input to the fiber amplifier in which the gain is clamped with the laser oscillation, even if the input optical signal is weak, the gain remains constant regardless of the intensity of the optical signal, and if the intensity of the input optical signal grows stronger, the laser oscillation is paused and the gain-clamped characteristic of the fiber amplifier disappears.
FIG. 1 shows an example of the conventional gain-clamped fiber amplifier using laser oscillation.
An optical fiber is used as a gain medium 1, and a pump light is supplied from a laser diode (LD) 3 through a wavelength division multiplexed (WDM) coupler 2. Using the coupler 4 at input and output stages, an optical attenuator (ATT) 5, an optical isolator (ISO) 6, and a transmission type optical filter (BPF) 7 constitute a ring cavity. The transmission type optical filter 7 adjusts a wavelength at which the laser oscillation occurs, the optical isolator 6 causes oscillation in the ring cavity to occur only in one direction, and the optical attenuator 5 adjusts the gain of the amplifier by controlling optical loss in the cavity.
In the fiber amplifier shown in FIG. 1, the intensity of the input optical signal and the intensity of the laser optical signal generated therein have a complementary relationship. In other words, when the intensity of the input optical signal is small, the intensity of the laser oscillated light is large, and as the intensity of the input optical signal grows larger, the intensity of the laser oscillated light grows smaller. Thus, even when the intensity of the input optical signal changes, the amplification ratio remains substantially constant. Here, when the intensity of the input optical signal grows to a certain level, the laser stops oscillating and the gain thereof decreases like a common fiber amplifier.
However, in the gain-clamped fiber amplifier using the laser cavity described above, when the intensity of the input optical signal varies, the intensity of the optical signal temporarily fluctuates due to a relaxation oscillation in the laser cavity. Such a temporary fluctuation in optical signal intensity has an effect on a bit error rate (BER) of transmitted data. In addition, a frequency of the relaxation oscillation depends on the gain medium and the length of the cavity, thereby making it difficult to fabricate the fiber amplifier and limiting an optical signal transmission rate and processing speed.
To solve this problem caused by the laser cavity, the gain-clamped fiber amplifier using an amplified spontaneous emission (ASE) reflector that causes ASE to be incident back upon the gain medium, rather than the laser cavity, has been developed.
The greater the intensity of the optical signal input for amplification, the lesser the intensity of the ASE. Further, the intensity of the ASE reflected by the reflector is proportional to the intensity of the ASE. In other words, when the intensity of the input optical signal is small, the intensity of the ASE becomes so large that the intensity of the ASE reflected by the reflector becomes large, while when the intensity of the input optical signal is large, the intensity of the ASE becomes so small that the intensity of the ASE reflected by the reflector becomes small. Consequently, the input optical signal and the reflected ASE are amplified in the gain medium, in which each contributes to the gain. Here, the intensity changes of two beams are opposite to each other, the gain remains almost the same until the intensity of the input optical signal grows to a certain level, and when the intensity of the input optical signal increases further, the gain of the amplifier is reduced. For convenience, when the clamped gain is lowered by 1 dB, the intensity of the input optical signal is defined as a dynamic range.
An ASE reflector gain-clamped Erbium-doped fiber amplifier (EDFA) using a reflector in which a mirror is coupled to a 1530 nm CWDM (Coarse WDM) is disclosed in Joon Tae Ahn, et al., “All-Optical Gain-Clamped EDFA With Improved Noise Figure and Freedom From Relaxation Oscillation”, IEEE Photonics Technology Letters, Vol. 16 No. 1, pp. 84-86, 2004. 1. In this amplifier, the stronger the reflected ASE, the wider the dynamic range. Thus, to obtain a strong reflected ASE, a wavelength of 1531 nm, at which the ASE is the strongest, was included, and 1530 nm CWDM having a transmission wavelength band of 1521 to 1539 nm was used to reflect the wavelength as wide as possible. An input optical signal having a wavelength of 1550 nm was used and the gain was measured, so that the gain-clamped characteristic could be obtained and the dynamic range was about 12 dBm. In addition, with respect to the switching characteristic according to the change of the input optical signal, the relaxation oscillation observed in the conventional laser oscillated gain-clamped fiber amplifier was not seen. However, this amplifier has a drawback in that the 1530 to 1540 nm wavelength portion of the Conventional band (C-band) defined as 1530 to 1565 nm is not amplified due to the 1530 nm CWDM used for the wide dynamic range.